1. Field of the Invention
The present invention relates generally to wastewater treatment systems and methods, and, more particularly, to such systems and methods using bioremediation techniques.
2. Related Art
Engineered wetlands for wastewater treatment are known to have three basic hydraulic configurations: surface flow (SF), subsurface horizontal flow (SSHF), and vertical flow (VF), the first two believed the most common, and are known to have significant design shortcomings. Even though an early wastewater treatment wetland design utilized vertical flow, design criteria are still considered experimental for vertical flow wetlands. Surface-loaded, vertical-flow wetlands are believed advantageous because surface loading forces flow through the root zone.
The basic hydraulic flow path for VF wetlands is for wastewater to be introduced at the wetland surface, pass through media and plant roots, then to flow out of the wetland via an underdrain system. Vertical flow wetlands are often designed to have a period of filling followed by a period of draining. When filled by wastewater, bacterial metabolism within the media depletes dissolved oxygen, producing anoxic or anaerobic conditions. As water drains, air is drawn down into wetland media, which is important to permit aeration of wetland media. Drain and fill cycles with a period of approximately a day or less are termed tidal flow. Previously known tidal flow systems are believed to have poor denitrification performance, with the exception of a reciprocating tidal flow system as taught by Behrends (U.S. Pat. No. 5,863,433).
Lagoon wastewater treatment systems comprise large basins in which wastewater is retained for many days or weeks. Depending upon organic mass loading and design, lagoons may be anaerobic, aerobic, or facultative. A facultative lagoon typically has an upper layer that is aerobic and a lower layer that is anaerobic, as a result of stratification. Typically cyanobacteria or algae dominate such lagoons. Aerobic and anaerobic cycling may be diurnal in nature, depending upon photosynthesis and wind-induced mixing.
The advantage of lagoons is their low capital and operating costs. However, lagoons demand large land footprints, owing to the long residence times; in addition, they are not capable of achieving advanced treatment, typically reaching secondary treatment standards at best. Algal growth in lagoons often creates effluent total suspended solids (TSS) concentrations that are much higher in biological oxygen demand (BOD) and TSS than secondary treatment standards would permit, and the filtration of algae from lagoon effluent is difficult.